Process of removing hydrolyzed polyacrylonitrile from an aqueous mixture

ABSTRACT

HYDROLYZED POLYACRYLONITRILE IS REMOVED FROM AN AQUEOUS MIXTURE CONSISTING ESSENTIALLY OF A QUATERNARY AMMONIUM SALT, ADIPONITRILE, ACRYLONITRILE AND HYDROLYZED POLYACRYLONITRILE BY CONTAINING THE MIXTURE AT A TEMPERATURE UP TO ABOUT 45*C. AND FOR AT LEAST ABOUT ONE HOUR TO FORM AN ORGANIC PHASE AND A WATER PHASE, THE PHASES SEPARATED BY AN INTERFACE (THE INTERFACE NOT BEING DEFINITELY DEFINED BUT CONSISTING ESSENTIALLY OF AN EMULSION WHICH CONSISTS ESSENTIALLY OF THE HYDROLYZED POLYACRYLONITIRLE), AND WITHDRAWING A PORTION OF THE INTERFACE AND FILTERING SAID PORTION TO REMOVE THE HYDROLYZED POLYARYLONITRILE.   D R A W I N G

United States Patent 01 ice 3,660,258 PROCESS OF REMOVING HYDROLYZED POLY- ACRYLONITRILE FROM AN AQUEOUS MIXTURE Donald E. Danly, Pensacola, Fla., and Andrew M. Patterson, Jr., Decatur, Ala., assignors to Monsanto Company, St. Louis, Mo. Continuation-impart of application Ser. No. 618,248, Jan. 30, 1967. This application Jan. 23, 1970, Ser. No. 5,440

Int. Cl. C07b 29/06; C07c 121/25, 121/28 US. Cl. 204-73 7 Claims ABSTRACT OF THE DISCLOSURE Hydrolyzed polyacrylonitrile is removed from an aqueous mixture consisting essentially of a quaternary ammonium salt, adiponitrile, acrylonitrile and hydrolyzed polyacrylonitrile by containing the mixture at a temperature up to about 45 C. and for at least about one hour to form an organic phase and a water phase, the phases separated by an interface (the interface not being definitely defined but consisting essentially of an emulsion which consists essentially of the hydrolyzed polyacrylonitrile), and withdrawing a portion of the interface and filterling said portion to remove the hydrolyzed polyacrylonitri e.

This application is a continuation-in-part of our copending application S.N. 618,248 entitled Process of Removing Hydrolyzed Polyacrylonitrile From an Aqueous Mixture, filed on J an. 30, 1967, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a process of removing hydrolyzed polyacrylonitrile from an aqueous mixture consisting essentially of a quaternary ammonium salt, adiponitrile, acrylonitrile and hydrolyzed polyacrylonitrile. More specifically, this invention relates to a process of removing hydrolyzed polyacrylonitrile from such an aqueous mixture by introducing the aqueous mixture into a decanter wherein the mixture is separated into an organic phase (comprising the adiponitrile and the acrylonitrile) and a water phase (comprising the quaternary ammonium salt), the phases separated by an interface (the interface consisting essentially of an emulsion containing the hydrolyzed polyacrylonitrile) and withdrawing a portion of the interface to separate the hydrolyzed polyacrylonitrile from the aqueous mixture.

Adiponitrile can be continuously produced by electrochemical dimerization of acrylonitrile to adiponitrile. This process is effected in an electrolytic cell containing at least one anode chamber and at least one cathode chamber, the chambers separated by an ion exchange membrane. In brief, the process comprises circulating an anolyte comprised of a dilute acid solution through the anode chamber, circulating a catholyte comprised of acrylonitrile dissolved in an aqueous quaternary ammonium salt solution through the cathode chamber, and establishing an electrical potential between the anode and the cathode sufficient to produce a unidirectional current flow and thus to reductively dimerize the acrylonitrile to adiponitrile at the cathode. The electrical potential causes the hydrogen ions of the dilute acid solution to migrate through the ion exchange membrane to make available the two hydrogen atoms required to dimerize acrylonitrile to adiponitrile. As the dimerization progresses a portion of the catholyte exiting from the cathode chamber is withdrawn and an equal portion of make-up quaternary ammonium salt solution containing acrylonitrile is added to the catholyte to maintain a constant volume catholyte stream.

3,660,258 Patented May 2, 1972 The portion of the catholyte which is withdrawn from the catholyte stream contains water, quaternary ammonium salt, adiponitrile, acrylonitrile and a small quantity of by-products, e.g., propionitrile, bis-cyanoethylether, hydroxypropionitrile, etc., and a small quantity of hydrolyzed polyacrylonitrile. Adiponitrile, the desired product, is separated from the catholyte and the quaternary ammonium salt is returned to the electrolytic cell in the catholyte stream.

The quaternary ammonium salt employed in the catholyte of the electrolytic cell must serve two functions, i.e., the salt must provide ions in aqueous solution for conducting an electrical current and the salt must be capable of increasing acrylonitrile solubility.

Included among the suitable quaternary ammonium salts are tetraalkyl ammonium sulfates, tetraalkyl ammonium aryl sulfonates, and tetraalkyl ammonium alkyl sulfates. Two specifically useful salts are tetramethyl ammonium toluene sulfonate and tetraethyl ammonium ethyl sulfate. Any presently known quaternary ammonium salt will serve the two functions above described. Any presently known quarternary ammonium salt, as an element of the catholyte stream, may be operated upon in accordance with this invention for the removal of hydrolyzed polyacrylonitrile. I

For etficient operation within the electrolytic cell, the quaternary ammonium salt in the catholyte stream must be kept substantially free of harmful impurities. Such a harmful impurity is hydrolyzed polyacrylonitrile.

By hydrolyzed polyacrylonitrile is meant polyacrylonitrile in which dependent-ON groups on the hydrocarbon backbone have been sufliciently hydrolyzed, i.e., to CONH -CONH and COOH groups, to result in at least partial solubility of the polymer in the electrolyte. The formation of such soluble polymeric material in the electrohydrodimerization of acrylonitrile and the undesirable results of the presence of such materials are Well known in the art.

For example, small amounts of this harmful impurity, e.g., about 0.1% based on the weight of the catholyte stream, can cause deposits on the cathode and, as a result, substantially reduce the efficiency of the electrochemical dimerization of acrylonitrile to adiponitrile.

Various methods have been proposed for commercial removal of such impurities from the aqueous quaternary ammonium salt solution which is withdrawn from the catholyte of the electrolytic cell during electrochemical dimerization of acrylonitrile. Filtration, of course, can be accomplished when the hydrolyzed polyacrylonitrile has been precipitated generally (as opposed to existence of a precipitate in an interface). An example of this method of removal is contained in US. Pat. No. 3,335,162. When removal is by general precipitation, it requires, of course, a filtration of the entire aqueous catholyte stream. Such filtration requires a great deal of time and effort, and can be pointed out as a serious bottleneck in commercial continuous production of adiponitrile.

SUMMARY OF THE INVENTION It is an object of this invention to provide a process of removing hydrolyzed polyacrylonitrile from an aqueous mixture consisting essentially of a quaternary ammonium salt, adiponitrile, acrylonitrile and hydrolyzed polyacrylonitrile.

Another object of this invention is to substantially reduce cathode fouling in an electrochemical cell used in the electrochemical dimerization of acrylonitrile to adiponitrile.

A still further object of the invention is to improve the efiiciency of the electrochemical dimerization of acrylonitrile to adiponitrile.

Other objects of this invention will become apparent as the invention is fully developed within the specification.

These and other objects of this invention are accomplished by providing a process of removing hydrolyzed polyacrylonitrile from an aqueous mixture consisting essentially of a quaternary ammonium salt, adiponitrile, acrylonitrile and hydrolyzed polyacrylonitrile comprising containing the mixture at a temperature up to about 45 C. and at a pH of from about 2.5 to about 7.5 for at least one hour to form a water phase and an organic phase, the phases separated by an interface, and withdrawing a portion of the interface as the hydrolyzed polyacrylonitrile. The interface is not a definitely defined interface but is an emulsion of the organic and water phase and consists primarily of hydrolyzed polyacrylonitrile.

It is the discovery that it is possible to obtain an interface containing essentially all of the hydrolyzed polyacrylonitrile in the mixture, that certain conditions are critical to the formation of this interface, and the practical use of the interface in removing this deleterious impurity from the catholyte stream by removal of the interface and filtration of this limited amount, which constitutes the essence of this invention. Conditions of pH and temperature are critical. However, as a practical matter the pH conditions are normally met by the catholyte stream in ordinary electrochemical dimerization process well known in the art. Temperature conditions, however, must be adjusted. Above temperatures of about 45 C., the hydrolyzed polyacrylonitrile will disperse throughout the aqueous layer and not be accumulated at the interface although the interface may exist at temperatures above 45 C.

On a practical basis, this process is useful for removing hydrolyzed polyacrylonitrile from an aqueous mixture consisting essentially of a quaternary ammonium salt, adiponitrile, acrylonitrile, and hydrolyzed polyacrylonitrile and also for separating therefrom and refining the quaternary ammonium salt, the acrylonitrile and adiponitrile, the process comprising:

(1) Introducing into a decanter an aqueous mixture consisting essentially of a quaternary ammonium salt, adiponitrile, acrylonitrile, and hydrolyzed polyacrylonitrile wherein the mixture is separated into an organic phase (comprising adiponitrile and acrylonitrile) and a water phase (comprising quaternary ammonium salt), the phases separated by an interface consisting essentially of an emulsion containing the hydrolyzed polyacrylonitrile;

(2) Withdrawing a portion of the water phase and washing said water phase with acrylonitrile to extract any adiponitrile in the water phase and passing the acrylonitrile plus any extracted adiponitrile into the organic phase of the decanter, the water phase consisting essentially of refined quaternary ammonium salt;

(3) Withdrawing a portion of the organic phase and washing said organic phase with water to extract any quaternary ammonium salt in the organic phase and passing the water plus any extracted quaternary ammonium salt into the water phase of the decanter, the organic phase being essentially refined and consisting essentially of adiponitrile and acrylonitrile; and

(4) Withdrawing a portion of the interface and filtering the interface portion to remove hydrolyzed polyacrylonitrile and passing the filtrate into the water phase of the decanter.

This practical aspect of the process provides for the removal of hydrolyzed acrylonitrile, for the refining and separation of the quaternary ammonium salt and combi nation of the adiponitrile and the acrylonitrile.

DESCRIPTION OF DRAWING The attached drawing shows the preferred embodiment of the process. It is to be understood that this drawing is not to be considered as a limitation of the invention, but is presented to show an assembly of equipment in which the process may be practiced.

Feed 2 of an aqueous mixture consisting essentially of a quaternary ammonium salt, adiponitrile, acrylonitrile and hydrolyzed polyacrylonitrile is passed into decanter 4 wherein the mixture is separated into an organic phase (comprising adiponitrile and acrylonitrile), a water phase (comprising quaternary ammonium salt), the phases separated by an interface (the interface not being definitely defined but consisting essentially of an emulsion of the organic phase and the water phase and consisting essentially of hydrolyzed polyacrylonitrile). The decanter is operated at a temperature up to about 45 C., and preferably from about 2035 C., and the hold-up time of the liquid in the decanter is at least one hour. A portion of the interface is withdrawn by stream 6, passed through filter 8 wherein the hydrolyzed polyacrylonitrile is removed and the filtrate 10 is passed into the water phase of decanter 4. A portion of the water phase in decanter 4 is withdrawn by stream 12, passed through acrylonitrile scrubber 28 wherein the water phase is scrubbed with acrylonitrile stream 16 to remove any adiponitrile or acrylonitrile, the scrubber water phase leaves scrubber 28 by stream 14 as an aqueous solution of refined quaternary ammonium salt, and the acrylonitrile plus any extracted adiponitrile leaves scrubber 28 by line 18 and is returned to the organic phase of decanter 4. A portion of the organic phase in decanter 4 is withdrawn by stream 20, washed with water in water scrubber 30 to remove any quaternary ammonium salt, the Wash water plus any extracted quaternary ammonium salt exits scrubber 30 by stream 26 and is returned to the water phase of decanter 4, and the washed organic phase leaves scrubber 30 by stream 22 as refined adiponitrile and acrylonitrib.

DESCRIPTION OF THE INVENTION As previously mentioned, hydrolyzed polyacrylonitrile is undesirable in the catholyte stream of an electrolytic cell wherein acrylonitrile is dimerized to adiponitrile. Such hydrolyzed polyacrylonitrile decreases the efiiciency of the dimerization by fouling the cathode and, if the hydrolyzed polyacrylonitrile is permitted to remain in the processing streams, it will form emulsions which adversely affect the extraction of the products in the exiting catholyte stream.

Hydrolyzed polyacrylonitrile can be removed from a catholyte stream, the catholyte stream consisting essentially of an aqueous mixture of a quaternary ammonium salt, adiponitrile, acrylonitrile and hydrolyzed polyacrylonitrile, by a process comprising containing the mixture at a temperature up to about 45 C. and for at least about one hour to form a water phase and an organic phase, the phases separated by an interface, and withdrawing a portion of the interface as the hydrolyzed polyacrylonitrile. Preferably, the mixture can be contained at a temperature within the range of from about 20-35 C. and for a time of at least about one hour. The organic phase comprises adiponitrile and acrylonitrile and the water phase comprises quaternary ammonium salt. The organic phase and the water phase is separated by the interface, the interface not being definitely defined but consisting essentially of an emulsion containing part of the organic phase and part of the water phase and consisting essentially of hydrolyzed polyacrylonitrile. Impurities, e.g., propionitrile, bis-cyanoethylether, hydroxypropionitrile, and other by-products obtained from the dimerization of acrylonitrile to adiponitrile are present in the organic phase and water phase, depending on the solubility thereof-generally speaking these impurities are not harmful to the process and are permitted to remain in the processing streams.

This being a physical separation phenomenon, the proportion of the constituents of the catholyte are not critical to this process for the removal of hydrolyzed polyacrylonitrile, except in the sense that if there should be less than about 0.01% based on the weight of the catholyte stream of hydrolyzed poyacrylonitrile in the catholyte stream, there will be insuthcient interface emulsion for a practical withdrawal and removal.

It is desired that the pH of the aqueous mixture in the decanter be within the range of from about 2.5 to about 6 amperes for the first hour, about 3.2 amperes for the second hour, about 3.4 amperes for the third hour, and an of 19.0 to 17.3 volts for the first hour and from 17.30 to 17.0 for the next two hours. At the end of the 7.5. Within this range the particle size of the hydrolyzed three hour period an electric current of 3.5 amperes was polyacrylonitrile is larger and thus facilitates filtration passed into the cell for an additional 30 minutes. The of the interface portion containing the hydrolyzed polyelectrolysis was thus conducted at from 5.5 to 6.2 amps./ acrylonitrile. More preferably, it is desired that the pH dm. and a total of 10.1 amp-hrs. The temperature of of the mixture be within the range of 3.5-6.5. the catholyte was maintaied at 2 0.5" C. to 26 C. by em- The hold-up time in the decanter should not be less 10 ploying a mixture of acetone and Dry Ice in the cooling than about one hour in order to effect adequate separajacket. During the electrolysis, a total of 3.0- ml. of glacial tion of the aqueous mixture into an organic phase and acetic acid was added to the catholyte to maintain the a water phase and to allow the hydrolyzed polyacrylonipH thereof just alkaline to phenol red. trile particles to move to the interface. The hold-up time Three portions of a catholyte stream obtained as above is unlimited, but due to the economics of the process a were introduced into separate decanters. minimum hold-up time is preferred. The conditions within the decanters, i.e., pH of the The following example is presented to illustrate the mixture, hold-up time of the mixture in the decanter, and invention. Where percents are used, it is based on weight temperature of the mixer in the decanter, are indicated unless otherwise specified. in Table I. The aqueous mixture separates into an aqueous phase and an organic phase and an interface consist- EXAMPLES ing essentially of an emulsion consisting essentially of Tetraethylammonium p-toluenesulfonate was prepared y i y p y ryl ni rile sep the tWO p A as follows; A mixture consisting f 20.0 (1 mole) f portion of the interface is withdrawn from the decanter ethyl p-toluenesulfonate, 101 g. (1 mole) or triethylamine and the amount of hydrolyzed polyacl'ylomtflle filtered and f absolute alcohol was stirred at room is indicated in Table I, the filtrate is returned to the aqueperature for 5 hours and then heated to 72 C i i ous phase in the decanter. Table I also indicates the per- 40 minutes. At this point an exothermic reaction occurred, cent hydrolyzed Polydcrylonitrile removed y the P and extraneous heating was discontinued and the mixture In yet another example, a catholyte stream from an allowed to stand for minutes. At the end of that time electrolytic hydrodimerization process similar to that of it was heated to reflux, and refluxing was continued for 30 Example 1, and utilizing the same materials and condi- 6 hours. After being allowed to cool to room temperations, but in commercial quantities, was subjected to the ture, the solvents and any unreacted material was stripped three-phase operation referred to in the drawing and in off with an aspirator to obtain a residue which solidified. the description of the drawing, this operation including This was washed with absolute ether three times by deconditions within the decanter as to pH, temperature and cantation. After removing traces of solvent from the holdup time identical with that of Example 3. Table II washed product with an aspirator, there was obtained as shows by material balance at points corresponding to the residue 296.8 g. of the substantially pure tetramethylamnumbers on the drawing, a virtually complete removal monium p-toluenesulfonate, M.P. 103-104" C. of polyacrylonitrile by the process of this invention.

TABLE 1 Pounds of hydrolyzed Hydrolyzed Temp. of Hold-up polyaerylonitrile polyacrylopH of aqueous time in nitrile Aqueous aqueous mixture decanter in aqueous Removed removed, mixture mixture C.) (hrs) mixture at filter percent No'rE.-'Ihese above data indicate that hydrolyzed polyaerylonitrile can be efleetively removed from the aqueous mixture.

TABLE IL-MATERIAL BALANCE SHOWING POLYACRYLONITRILE REMOVAL FROM AN AQUEOUS MIXTURE [Pounds/hour] Organic Interface Filter phase to Acrylonitrile Decanter to filter efliuent scrubber scrubber Component feed (2) (6) (10) (20) solvent (16) Tetraethylammonium p-toluenesulionate. 24, 696 2,021 1, 971 1, 376 Water 24, 643 1, 708 1, 451 0 Acrylonitrile 11, 598 926 909 14, 783 Adiponitril 12, 045 1, 043 1, 024 7, 443 Polyacrylonit 78 78 0 O A concentrated aqueous solution (4 Baum) was pre- We claim:

pared by dissolving the 296.8 g. of tetraethylammonium p-toluenesulfonate in 230.5 g. of water. This was employed in the electrolytic hydromerization of acrylonitrile, as follows:

Acrylonitrile (94.5 g.) containing a trace of p-nitrosodimethylaniline as stabilizer was added to 141 g. of said solution of the sulfonate to give a catholyte containing 40% by weight of acrylonitrile, based on the total weight of the catholyte. A platinum anode was placed in an Alundum cup containing, as anolyte, 30 ml. of said concentrated aqueous solution of the tetraethylammonium p-toluenesulfonate and immersed into a jacketed glass vessel containing the catholyte and 110 ml. of mercury as cathode. An electric current was then passed through the resulting cell for three hours at about 2.3

1. A process of removing hydrolyzed polyacrylonitrile from an aqueous mixture consisting essentially of a quaternary ammonium salt, adiponitrile, acrylonitrile and hydrolyzed polyacrylonitrile at a pH of from about 2.5 to about 7.5 comprising containing the mixture at a temperature up to about 45 C. for at least about one hour to form a water phase and an organic phase, the phases separated by an interface, and withdrawing a portion of the interface as the hydrolyzed polyacrylonitrile.

2. The process of claim 1 wherein the temperature of the mixture is from about 20 to about 35 C.

3. A process of claim 1 wherein the pH of the mixture is from about 3.5 to about 6.5.

4. In a process for the continuous production of adiponitrile by electrochemical dimerization of acrylonitrile to adiponitrile, wherein said dimerization takes place in an electrolytic cell containing at least one anode chamber and at least one cathode chamber, said chambers being separated by an ion exchange membrane, and wherein an anolyte comprised of a dilute acid solution is circulated through the anode chamber, and a catholyte comprised of acrylonitrile dissolved in an aqueous quaternary ammonium salt solution is circulated through the cathode chamber, and wherein an electrical potential is established between the anode and cathode suflicient to produce a unidirectional current flow and thus to reductively dimerize the acrylonitrile to adiponitrile at the cathode, and wherein the electrical potential causes the hydrogen ions of the dilute acid solution to migrate through said ion exchange membrane thereby making available hydrogen atoms which dimerize acrylonitrile to adiponitrile, and wherein as dimerization progresses, a portion of catholyte is withdrawn from said cathode chamber being concomitantly replaced by new catholyte, the improvement comprising:

(1) introducing the catholyte into a decanter,

(2) maintaining the catholyte in the decanter at a temperature of from about 20 C. to about 45 C. and at a pH of from about 2.5 to about 7.5 for a period of at least one hour, wherein the catholyte separates into an organic phase and a water phase, the phases separated by an interface; and

(3)withdrawing a portion of the interface.

5. The process of claim 4 wherein the pH of the aqueous mixture in the decanter is within the range of from about 3.5 to about 6.5.

6. In a process for the continuous production of adiponitrile by electrochemical dimerization of acrylonitrile to adiponitrile, wherein said dimerization takes place in an electrolytic cell containing at least one anode chamber and at least one cathode chamber, said chambers being separated by an ion exchange membrane, and wherein an anolyte comprised of a dilute acid solution is circulated through the anode chamber, and a catholyte comprised of acrylonitrile dissolved in an aqueous quaternary ammonium salt solution is circulated through the cathode chamber, and wherein an electrical potential is established between the anode and cathode sufiicient to produce a unidirectional current flow and thus to reductively dimerize the acrylonitrile to adiponitrile at the cathode, and wherein the electrical potential causes the hydrogen ions of the dilute acid solution to migrate through said ion exchange membrane thereby making available hydrogen atoms which dimerize acrylonitrile to adiponitrile, and

wherein as dimerization progresses, a portion of catholyte is withdrawn from said cathode chamber being concomitantly replaced by new catholyte, the improvement comprising:

(1) introducing the catholyte mixture into a decanter wherein said mixture is maintained at a temperature up to about 45 C. for at least one hour and wherein the mixture is separated into an organic phase and a water phase, the phases separated by an interface and the organic phase comprising adiponitrile and acrylonitrile, the water phase comprising quaternary ammonium salt and the interface consisting essentially of hydrolyzed polyacrylonitrile.

(2) withdrawing a portion of the water phase and washing said water phase with acrylonitrile to extract any adiponitrile in said portion of water phase and passing the acrylonitrile plus any extracted adiponitrile into the organic phase of the decanter;

(3) withdrawing a portion of the organic phase and washing said portion of the Organic phase with water to extract any quaternary ammonium salt in the organic phase and passing the water plus any extracted quaternary ammonium salt into the water phase of the decanter; and

(4) withdrawing a portion of the interface and filtering the interface portion to remove hydrolyzed polyacrylonitrile and passing the filtrate into the water phase of the decanter.

7. The process of claim 6 wherein the temperature of the aqueous mixture in the decanter is within range of from about 20 C. to about 35 C.

References Cited UNITED STATES PATENTS 3,484,348 12/1969 Johnson et a1 204-73 A 3,245,889 4/1966 Baizer 20473 A 3,497,429 2/ 1970 Mihara et a1. 20473 A 3,267,131 8/1966 Campbell et al 20473 A 3,335,162 8/1967 Campbell et a1 260465.8 3,493,597 5/1967 Campbell et a1 260-459 3,597,331 8/1971 Ogawa et a1. 203-43 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 

